This relates to silicon compounds, including silicon nitride, silicon carbide and silicides, and processes for production thereof.
Silicon nitride has the useful properties of very high tensile strength, mechanical hardness, high melting point, high chemical resistance, high refractive index and good electrical insulating properties. It is commonly used as a protective coating for high performance mechanical parts and as a thin, dielectric film in microelectronic and optical components. The amorphous compound Si.sub.3 N.sub.4, is most often used, of which there are many different microstructural forms, e.g. .alpha., .beta. and gamma.
Bulk silicon nitride is usually formed by sintering powdered silicon nitride at high temperatures, greater than 1000.degree. C., and under high pressure. It can also be deposited as a thin film by sputtering from a solid target, or by chemical vapor deposition (CVD). One example of the latter is the decomposition of silane and ammonia over the surface of a heated substrate.
CVD processes allow the formation of conformal silicon nitride thin films with highly controlled composition, and low defect density. The lower the number of defects and impurities in the film, the better the dielectric properties, chemical resistance, and hardness of the material. However, CVD films have high residual stress (2.times.10.sup.9 Pa, tensile stress for Si.sub.3 N.sub.4), which limits the deposition thickness to less than about one micron if microcrack formation is to be avoided. Although sputtered films have much lower residual stress and can therefore be deposited to greater thicknesses, sputtering is a slow process, most sputtering systems have a lower substrate capacity than CVD systems, and sputtered films have higher defect density and impurity concentrations than CVD films.
Thin films of very high quality silicon nitride can also be produced by direct nitridation of silicon. Silicon substrates or films are heated and reacted with ammonia and/or nitrogen. The film growth quickly becomes limited by the diffusion of reactant through the growing film. The film thickness can be increased by increasing the growth temperature, but, even at 1100.degree. C., it is difficult to produce films thicker than about 100 .ANG.ngstroms.
Silicon nitride possesses a unique combination of properties that makes it an excellent fabrication material for micro- mechanical, electrical and optical components. Applications include situations where thick films, or plates, of silicon nitride would be desirable, for example, as a mask for x-ray lithography for integrated circuit fabrication. Substrates on which the masking, i.e. x-ray absorbent, patterns are formed, are made x-ray transparent by the appropriate combination of material and its thickness. Thin plates of silicon can be used, but silicon must be made very thin in order to transmit x-rays, and this results in very fragile masks and limited mask size. Silicon nitride is more X-ray transparent and a much stronger material. However, present methods of making silicon nitride limit its use due to high residual stress. Silicon-rich nitrides have lower residual stress, but also have lower transmittance. Accordingly, a means to form thick, i.e., greater than about five microns, films of silicon nitride with moderate tensile stress would be very useful in the future development and commercialization of x-ray lithography.
Another potential application of low stress, thick silicon nitride is as components of micromechanical systems. The growing field of micro-mechanics (the micro-motor) is creating a demand for materials which can be micro-machined and can withstand repeated strain and frictional wear. Silicon nitride is one of the hardest materials known and is more chemically stable than silicon; it is therefore an excellent material for use in such applications.
Silicon rich nitride has a much lower stress than stoichiometric nitride, which allows its use in thick films of up to about 5 microns. Unfortunately, silicon rich nitride also has less chemical resistance, hardness and optical transparency.
The advantages of stoichiometric nitride over the silicon rich nitrides currently used in these and other designs, include a higher chemical resistance and better electrical isolating properties. The present technologies for forming silicon nitride films of 3:4 stoichiometry result in films of very high residual stress, limiting their thicknesses to less than about 1 micron. Thick films of silicon nitride, of nearly stoichiometric composition, which could be formed on a silicon substrate, have several immediate, and potential, applications. One is the formation of silicon nitride membranes, diaphragms and plates on silicon wafers. Membranes and diaphragms have potential applications as micromechanical components, for example as the pressure deformable component of a micro pressure transducer. Another potential application is an optical window for integrated optics and memory chips. Further, silicon nitride could be used as a thermally insulating membrane in microsensor configurations which employ heaters, such as semiconductor gas sensors which rely on thermal desorption characteristics of gases for selectivity, or in flow sensors which rely on the thermal conductivity of gases, etc.
The choice of a fabrication material is determined not only by its performance in the intended role, but also by consideration of the ease by which it can be manufactured. Silicon nitride has the added advantage of high chemical resistance, which makes it an excellent etch-mask for silicon anistropic etchants. It can also be used as an etch stop layer. Since it is resistant to corrosion, and an excellent barrier to the diffusion of ions and water, integrated circuit manufacturers have taken advantage of these properties and use silicon nitride as a final encapsulating layer. It has also been extensively used as a solid state encapsulant for chemical microsensors.
Chemical resistance is particularly useful in the fabrication of nitride membranes, because it facilitates the selective removal of the silicon substrate, or intermediate layers, from the nitride. Also, silicon nitride is often used as an etch stop layer in the surface micromachining processes which are used to release thin film micromechanical parts from the substrate onto which they have been deposited. Nitride is particularly useful when the spacer or sacrificial layer between the thin film mechanical parts and the substrate is silicon oxide. This is because the etchant used for release, i.e. to remove the oxide, contains hydrofluoric acid, which etches stoichiometric nitride at a rate somewhat less than 1/100 the rate it etches oxide. The long etch times for releasing parts results in a need for very thick nitride stop layers. Currently, these layers are limited to less than a micron, and hence the etch time, and the geometries of structures that can be released, are also limited.
Additional microfabrication and device applications also exist with respect to other silicon compounds, such as silicon carbide, and silicides such as TiSi and WSi. In contrast to silicon nitride and silicon carbide, silicides are conductors and therefore have applications distinct from those utilizing the insulating properties of the silicon nitride and silicon carbide.
It is therefore an object of the present invention to provide low stress, low or defect free, silicon nitride films of uniform thickness of greater than about one micron.
It is a further object of the present invention to provide a method of stabilizing porous silicon morphologies with a silicon nitride layer.
It is a further object of the present invention to provide porous silicon nitride layers.
It is another object of the invention to provide porous silicon carbide and silicide layers.